1. Field of the Invention
The present invention relates to jack-up or self-jacking platforms for oil drilling and more particularly relates to platforms for oil drilling and/or oil production operations at sea.
2. Description of the Prior Art
Jack-up platforms for off-shore drillings have been used until recently for depths of less than 100 m.
The use of such platforms in difficult seas (North Sea, East Canada, etc.) for depths exceeding 100/110 m with 30 m high waves is rendered difficult:
by the dynamic behaviour of the platform resulting from the great extent of the support legs;
by the forces to which the platform is subjected on the part of the swell, the wind, the currents.
Some constructors propose designs which retain the conventional architecture of jack-up platforms for reaching depths exceeding 100/110 m. This results in a large increase in the dimensions of the legs and the hull and in the displacement when towing.
Legs of the lattice type are obtained with spacings between the members which may exceed 20 m.
The increase in the dimensions of the legs results in a large increase in the weight, a higher center of gravity when towing and therefore a loss of stability, and greater drag forces when towing.
Apart from the economic aspect, this renders the transfer of such a platform from one drilling site to another difficult.
Further, a jack-up platform does not permanently work at the maximum depth it is capable of reaching.
Consequently, a jack-up platform designed for a depth of 130 m will often be made to work at depths of 100 m and less and it is then faced with competition from smaller platforms which are easier to handle and cheaper to make.
Drilling operators prefer working with self-jacking platforms which are, in the high drilling position on their legs, "fixed" platforms, rather than with semi-submersible platforms which are mobile platforms.
Thus, at the present time, for depths of less than 80 m, jack-up platforms are almost exclusively employed.
Current constructions propose pushing back the limit of utilization of jack-up platforms to a depth of 100/110 m in difficult seas.